Transport of nitrate in the Mississippi river in July-August 1999

Lagrangian sampling was conducted on the Mississippi River in late July through early August 1999 to test the hypothesis that nitrate (NO₃^-) is transported conservatively in the Mississippi River. Three different approaches were pursued to test the hypothesis: (1) a mass balance for NO₃^- was evaluated for evidence of net gains and losses, (2) stable isotopes of NO₃^- were measured (δ¹⁵N and δ¹⁸O) to determine if fractionation occurred, and (3) the concentrations of dissolved gases (N₂O, N₂ and Ar) in river water were measured and compared to theoretical equilibrium concentrations. Integrated water samples and flow measurements were obtained at 10 sites on the Mississippi River and 7 sites near the mouths of major tributaries from northern Iowa to southern Louisiana, a distance of about 2,250 river kilometers. Mass balance calculations indicate that more than 80 percent of the NO₃^- mass discharged from the Mississippi River (1,930 metric tons/day) during the study period originated in the first 500 river kilometers of the study reach. The mass balance calculations also indicate that NO₃^- was not lost from the water column upstream of Vicksburg, MS, but that there might have been some loss of NO₃^- in the lower 700 kilometers of the study reach. The stable isotope ratios of N and O (δ¹⁵N and δ¹⁸O) of NO₃^- were consistent with mixing and transport in the absence of fractionating gains or losses. The concentrations of nitrogen (N₂) and argon (Ar) dissolved in river water decreased in the downstream direction, approximately in equilibrium with air at increasing temperatures, giving no evidence of gains or losses of N₂ by nitrogen fixation or denitrification. Nitrous oxide (N₂O) concentrations in the Mississippi River were approximately 26 to 200 percent of air saturation, indicating relatively low net production by combination of nitrification and denitrification. Results from this study indicate that most (>90%) of the NO₃^- that entered the Mississippi River during July-August 1999 was transported to the Gulf of Mexico.